Patent GB-215164-A (GB)
Title: Improvements in signalling systems
IPC: H04K1/04
Filed: 1923-03-29
Abstract: 215,164. Western Electric Co., Ltd., (Western Electric Co., Inc.). March 29, 1923. Secret and selective signalling systems; distortion of voice currents, effecting and neutralizing; wired wirelesssystems.-In order to secure secrecy in telephony, the speech currents are distorted, as by inverting their frequencies, and are then combined by modulation with a wave, the frequency of which is periodically varied. Owing to these frequency variations, the message cannot be received by simple homodyne or heterodyne reception. When wireless transmission is employed, the message is received by first rectifying, and then compensating for the speech distortion, as by re-inverting the speech frequencies. For the reception of wire telephony, the frequency of a local oscillator is varied synchronously with the transmitter, by means of synchronizing impulses sent over the line. Figs. 3 and 4 show respectively the transmitter and receiver circuits for wireless telephony. The microphone current is amplified and applied to a balanced modulator 54 which is also fed by a source 55 of frequency k, beyond the range of essential speech frequencies. An output filter 56 is designed to suppress the upper side band resulting from the modulation, and this current is used to modulate the carrier frequency in the modulator 58. The carrier wave is supplied by a source 61 and has its frequency varied in an erratic manner, for example by a motor 63 rotating irregularly-shaped plates of a small condenser 62. The modulated carrier wave is passed through an amplifier 59 to the aerial 60. At the receiving station, the waves are received on the aerial 64 and passed through a rectifier 65. The small variation in carrier frequency produces no substantial effect, and the rectified current is applied to a modulator 67 by which it is re-combined with current of frequency k from a source 68. The resulting speech currents pass through a filter 69 to the telephone 70. In order that the currents in the receiving circuit 66, resulting from simple rectification by the detector 65, may agree with those in the transmitting circuit 57, it is necessary that the modulator 58 should be designed to pass the unmodulated carrier wave as well as the side bands. Synchronizing apparatus may, however, be employed to enable a local source at the receiving station to supply oscillations varied in frequency in exact correspondence with the variations effected at the transmitter by the condenser 62. The simple rectifier 65 would then be replaced by a homodyne receiver, and transmission of the unmodulated carrier wave would be necessary. Such synchronizing apparatus may readily be employed on wire circuits, as shown in Fig. 5, in which both transmitting and receiving circuits are shown connected to the line 43. The microphone circuit 30 is connected through a low-pass filter 32 to a modulator 33 in which the speech currents s are combined with current of frequency p (of the order of 10.000-50,000) from a source 38. The upper side band p+s is selectively transmitted by the filter 34, and is combined in the modulator 35 with current from the oscillator 39. This oscillator has its frequency continuously varied by a motor 42 rotating shaped condenser plates 41, so that the frequency is varied from a fixed minimum p+v to a variable frequency p+v+d, the maximum value of d being less than the lowest essential speech frequency, say 200, and v being chosen at approximately the highest essential speech frequency, 2200. The filter 36 in the output circuit of the modulator 35 is designed to pass only the lower side band or difference frequency v+d-s, and this band of speech frequencies is passed to the line 43. The motor 42 drives an impulse sender 44 by which synchronizing impulses are sent to line through a filter comprising shunt capacity 49 and series inductance 48. At the other end of the line 43, these impulses act in known manner to maintain the rotary shaft of a variable capacity, similar to 41, in synchronism with the shaft of the motor 42. With this exception, the apparatus and circuits at the other end of the lines arei identical with those shown in Fig. 5. The received band v+d-s passes through a filter 81 and amplifier 82 to a modulator 83 in which it is combined with the current of variable frequency p+v+d from the oscillator 39. The difference frequency band p+s is selectively transmitted by a filter 84 to a modulator 85 in which it is combined with current of frequency p from the source 38, giving rise to the original speech currents. The sources 38 at the two stations may be independent thermionic oscillators, or alternators driven bv the svnchronized motors 42. In a modified system of reception, the receiver band v+d-s is first combined with waves of frequency p to produce the band p+v+d-s, which is then combined with the variable waves of frequency p+v+d to produce the speech band s.